The prior art for electromagnetic drillstring telemetry is based upon inductive (toroidal) or direct coupling of a source signal carrying the downhole sensor information to the drillstring and surrounding formation. Toroidal coupled systems induce a modulated electric current on the drillstring by means of electromagnetic coupling between a (primary) toroidal coil encircling a conductive mandrel connected to the drillstring, and a secondary coil comprising the drillstring, and surrounding formation. The modulated current, which is induced in the secondary, flows along the drillstring and drilling fluid, and through the formation in a pattern, which is governed by the electrical conductivity(s) of the drillstring and drilling fluid, and surrounding formation. The flow of current on the drillstring and through the formation is measured by a receiving apparatus at the surface.
The receiving apparatus is either inductively coupled to the modulated current through a transformer or directly coupled by sensing the potential difference (voltage) produced by the flow of modulated current between electrodes xe2x80x9cgroundedxe2x80x9d at the surface. A previous patent (U.S. Pat. No. 4,181,014 to Zuvela et al) describes several means of signal reception using sub-surface electrodes connected to the surface by insulated conductors. (See also U.S. Pat. No. 4,980,682 to Klein et al).
The operation of the inductively coupled (toroidal) downhole transmitter-receiver (transceiver) is enhanced by insulating gaps in the downhole transceiver sub-assembly to isolate the toroidal primary coil from the surrounding drill collar (which would otherwise provide a direct short to the secondary, if it were not electrically isolated). The toroidal-inducing coil encircles an electrically conducting mandrel, which is mechanically and electrically connected to the upper and lower sections of drillstring. The toroidal sub-assembly and associated electronics are designed to provide impedance matching between the source circuitry and the load of the drillstring-formation circuit (U.S. Pat. No. 4,496,174 to McDonald et al, 1985).
In the prior art, the source impedance may be matched with the load using matching transformers (U.S. Pat. No. 2,389,241 to Silverman, 1944; U.S. Pat. No. 4,691,203 to Rubin, 1987). Matching transformers and associated complex electrical circuitry are employed to match the impedance of the downhole sub-assembly electronics to the very low impedance associated with the small gaps necessary to maintain the mechanical stability of the downhole transceiver sub-assembly. One of the herein inventors has previously patented an apparatus for electromechanical impedance matching (U.S. Pat. No. 5,130,706 to Van Steenwyk, 1992).
Transformer coupled electric-field telemetry systems require that the signal information be transmitted by various forms of modulation of a carrier signal. Pulse modulated systems have been described (U.S. Pat. No. 3,046,474 to Arps, 1962; U.S. Pat. No. 4,015,234 to Krebs, 1977); but these systems have required the generation of a very high-voltage pulse by means of capacitor discharge to overcome the poor impedance match between the downhole transmitter and the drillstring-formation load impedance.
More recently, a low-voltage, low-impedance, current generator has been described (U.S. Pat. No. 5,270,703 to Guest). It should be noted that none of these methods for coupling a pulse to the drillstring-formation path are suited to a talk-down capability. See also U.S. Pat. No. 4,684,946 to Geoservice.
The present invention relates to a method and apparatus to improve the effectiveness of electric-field borehole telemetry. A direct-coupled electromagnetic telemetry system is provided in which the downhole source drives a modulated electric current directly into the underground formation by means of a modulated voltage or current applied across an electrically insulating gap created in the drillstring by one or more gap sub-assemblies.
Another aspect of the invention is directed to the use of insulating drill collars and wireline components, to match the downhole impedance of electric signal transmitter circuitry to the electrical impedance of the surrounding drilling fluids and geologic formations. By means of this aspect of the invention, downhole power requirements can be significantly reduced.
Another feature of the invention is the use of the downhole electric fields generated by the telemetry apparatus for formation resistivity and induced polarization measurements. By using insulating drill collars and wireline components to vary transmitter and receiver electrode spacing and configuration, many of the methods of surface resistivity and induced polarization available to surface geophysics can be deployed on the drillstring, in conjunction with a downhole electric field telemetry system.
The invention provides a method and apparatus to configure an insulating gap in a drillstring or borehole casing, so as to enable the generation or detection of electric fields on the surface of the drillstring or borehole casing. The method can be used in the transmission of downhole measurements and drilling parameters from the drillstring to the surface, the transmission of control signals from the surface to a point on the drillstring, and the evaluation of resistivity and induced polarization response of the formation surrounding the drillstring, formation at the bit, or formation surrounding a cased borehole.
These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following specification and drawings, in which: